Thermal spraying, also known as flame spraying, involves the melting or at least heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. A thermal spray gun such as described in U.S. Pat. No. 3,455,510 (Rotolico) is used for the purpose of heating and propelling the particles. In this type of thermal spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are comprised typically of small particles, e.g., between 100 mesh U.S. Standard screen size (149 microns) and about 2 microns. Heat for powder spraying is generally from a combustion flame or an arc-generated plasma flame. The carrier gas, which entrains and transports the powder, may be one of the combustion gases or an inert gas such as nitrogen, or it simply may be compressed air.
Quality coatings of certain thermal spray materials have been produced by spraying at high velocity. Plasma spraying has proven successful with high velocity in many respects but it can suffer from non-uniform heating and/or poor particle entrainment which must be effected by feeding powder laterally into the high velocity plasma stream.
Rocket types of powder spray guns recently became practical and are typified in U.S. Pat. No. 4,416,421 (Browning). This type of gun has an internal combustion chamber with a high pressure combustion effluent directed through a nozzle or open channel. Powder is fed into the nozzle chamber to be heated and propelled by the combustion effluent.
Short-nozzle spray devices are disclosed for high velocity spraying in French Patent No. 1,041,056 and U.S. Pat. No. 2,317,173 (Bleakley). Powder is fed axially into a melting chamber within an annular flow of combustion gas. An annular air flow is injected coaxially outside of the combustion gas flow, along the wall of the chamber. The spray stream with the heated powder issues from the open end of the combustion chamber.
Sprayweld alloys of the boron-silicon-nickel type are known in the art, and comprise a nickel base to which is added relatively small percentages of boron and silicon to improve the fluxing characteristics of the nickel base alloy. Other elements, such as chromium, are frequently included in nickel base alloys of this type.
Such alloys are used for welding and brazing and particularly for coating materials applied as a fused or welded over-lay on base materials, such as steel or steel alloys. The elements boron and silicon, when added to nickel or nickel base alloys, act as fluxer of the alloy and of the surface to be alloyed during the fusing of the alloy when performing the brazing, welding or coating operation. For this reason such alloys are known as "self-fluxing alloys".
One process frequently used for applying fused coatings of boron-silicon-nickel alloys is known as "spray-welding". Spray-welding comprises the steps of first metal spraying the alloy onto the surface to be coated such as with a combustion powder spray gun of the type disclosed in the aforementioned U.S. Pat. No. 3,455,510, and second, fusing the coating in place. The metal spraying operation can be carried out by any of the known metal spraying techniques, in which the material to be sprayed is fed into a heating zone where it is melted or heat-softened and from which it is, in finely divided form, propelled in molten or heat-plastic condition onto the surface to be coated. After coatings have been applied by the metal spraying process, they are thereafter fused in the carrying out of the spray-welding process. Such fusing may be done in a furnace or, alternatively, by means of heating torches applied directly to the coated surface. Self-fluxing alloys and typical compositions are described in more detail in U.S. Pat. No. 2,875,043 (Tour).
Although fused coatings of self-fluxing alloys are quite wear resistant per se, further wear resistance is gained by blending hard particles such as a carbide powder with the alloy powder prior to spraying. Such a carbide generally includes a metal binder, such as tungsten carbide in a cobalt matrix as disclosed in British Patent No. 867,455. Fused thermal sprayed self-fluxing alloy coatings, with or without carbide, are susceptible of being ground and polished to a very high finish.
A particular application requiring the wear resistance and finish of such coatings is the production of glass mold plungers. In the manufacture of glass objects such as bottles an early step is to inject a heated rod-shaped plunger into a small mass of heat softened glass to produce an initial hole therein. The glass is thereafter blown into shape with compressed air applied into the hole. It is critical that the plunger have a smooth shape and a high, mirror-like finish in order to prevent flaws from developing in the glass objects. Even a small imperfection in the plunger surface picks up glass to form larger imperfections in subsequent operations with the plunger.
Because of wear and finish capability, thermal spraying has been used for many years for making and refurbishing glass mold plungers. U.S. Pat. No. 4,382,811 (Luscher et al.) teaches such utilization. Although oxide powder blended with the self-fluxing alloy is taught therein, self-fluxing alloy with or without carbide is much more commonly used on plungers.
However, a continuing problem is that it is quite difficult and technique dependent to thermal spray and fuse coatings onto glass mold plungers without flaws. Spray parameters are critical, requiring close control of gas flows, spray distance, traverse rates, spray speed and temperature control. Also, in finishing the coating, the tapered shape of the plunger results in overheating of the tip of the plunger. As a result only certain operators are sufficiently proficient to do the job, so it is expensive and has a high reject rate.
Therefore, objects of the present invention are to provide an improved method of manufacture of glass mold plungers, to provide an improved thermal spray method for manufacturing glass mold plungers, and to provide for the manufacture of glass mold plungers with reduced technique dependence, rejection rate and cost.